Re: [Xenomai-help] RTDM mmap alternative

[Jan Kiszka <jan.kiszka@domain.hid>]

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Rodrigo Rosenfeld Rosas wrote:
> Em Quarta 08 Fevereiro 2006 11:14, Jan Kiszka escreveu:
> 
>>> ...
>>> Jan, actually I didn't like the /dev/zero hack. If someone else is
>>> using /dev/zero it will result in a problem... I think it would be better
>>> to
>> Why? It isn't an exclusive device. And I'm only invoking open/close on
>> the original instance (which even have no handlers associated, see
>> drivers/char/mem.c), mmap gets overloaded.
> 
> I'm a little confused here. I noted you have made a backup of the 
> file_operations struct and private_data. So, I've concluded, wrongly, that 
> those handlers would be used by every process that called that open handler. 
> But, looking at the mem.c code, I think that those data (file_ops and 
> private_data) are different structs each time the user issues a call to open. 

Yep, that's the point. Actually, I was inspired by some other mmap'ing
driver which does a similar overloading on it's own device type.

> If that is true, I don't see any problem in using your approach. But I don't 
> see neither any needs for making that backup...

On first sight this is true and the backup is redundant. But it is
cleaner to not rely on a specific kernel implementation. Instead it's
saner to look at the model: I'm changing the fops without knowing what
the open handler did, so make sure that a possible release handler can
run. And this is also not very costly here. :)

Jan


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